Reflective cholesteric liquid crystal display with complementary light-absorbing layer

ABSTRACT

A display sheet comprising a substrate carrying layers of material; including a polymer-dispersed cholesteric liquid-crystal layer having a first high reflection state within a portion of the visible light spectrum and a second less-reflective state in said spectrum, said states being changeable by electric field between the two states which states can be maintained in the absence of an electric field; a first transparent conductor disposed over the polymer-dispersed cholesteric liquid-crystal layer; a complementary light-absorbing layer below the polymer-dispersed cholesteric liquid-crystal layer having relatively high light absorption within the spectrum of the high-reflection state of the polymer-dispersed cholesteric liquid-crystal layer and having relatively less light absorption in the spectrum complementary to that of the high reflection state of the polymer-dispersed cholesteric liquid-crystal layer; and a reflective second conductor under said complementary light-absorbing layer reflecting light received from the complementary light-absorbing layer back through the complementary light-absorbing layer.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned U.S. patent application Ser. No.09/799,220 filed Mar. 5, 2001, U.S. patent application Ser. No.09/915,441 filed Jul. 26, 2001, U.S. patent application Ser. No.10/036,149 filed Dec. 26, 2001.

FIELD OF THE INVENTION

The present invention relates to a display sheet having a cholestericliquid-crystal layer that can change states, a relatively lighter(brighter) state and a relatively darker state, to provide a viewableimage. In particular, the invention relates to a display sheet in whichthe relatively lighter state exhibits a more neutral appearance.

BACKGROUND OF THE INVENTION

Currently, information is displayed using assembled sheets of papercarrying permanent inks or displayed on electronically modulatedsurfaces such as cathode ray displays or liquid crystal displays.Printed information cannot be changed. Electrically updated displays areheavy and expensive. Other sheet materials can carry magneticallywritten areas to carry ticketing or financial information, howevermagnetically written data is not visible.

Media systems exist that maintain electronically changeable data withoutpower. Such system can be electrophoretic (Eink), Gyricon or polymerdispersed cholesteric materials. An example of electronically updateabledisplay can be found in U.S. Pat. No. 3,600,060 issued Aug. 17, 1971 toChurchill that shows a device having a coated then dried emulsion ofcholesteric liquid crystals in aqueous gelatin to form a fieldresponsive, bistable display. U.S. Pat. No. 3,816,786 discloses a layerof encapsulated cholesteric liquid crystal responsive to an electricfield. The electrodes in the patent can be transparent ornon-transparent and formed of various metals or graphite. It isdisclosed that one electrode must be light absorbing and it is suggestedthat the light absorbing electrode be prepared from paints containsconductive material such as carbon.

Fabrication of flexible, electronically written display sheets isdisclosed in U.S. Pat. No. 4,435,047 issued Mar. 6, 1984 to Fergason. Asubstrate supports a first conductive electrode, one or more layers ofencapsulated liquid crystals, and a second electrode of electricallyconductive ink. The conductive inks form a background for absorbinglight, so that the display areas appear dark in contrast to non-displayareas. Electrical potential applied to opposing conductive areasoperates on the liquid crystal material to expose display areas. Becausethe liquid crystal material is nematic liquid crystal, the displayceases to present an image when de-energized.

The Fergason patent discloses the use of nematic liquid crystal, whichabsorbs light and does not maintain an image in the absence of a field.Dyes in either the polymer encapsulant or liquid crystal are used toabsorb incident light. The dyes are part of a solution, and not solidsub-micrometer particles. The patent further discloses the use of achiral dopant in Example 2. The dopant improves the response time of thenematic liquid crystal, but does not operate in a light-reflectivestate.

U.S. Pat. No. 5,251,048 discloses a light-modulating cell having apolymer-dispersed chiral-nematic liquid crystal material. Thechiral-nematic liquid-crystal material has the property of beingelectrically driven between a planar state reflecting a specificspectrum of visible wavelength of light and a light scatteringfocal-conic state. Chiral-nernatic liquid-crystals, also referred to ascholesteric liquid crystals, have the capacity of maintaining (in astable state) one of a plurality of given states in the absence of anelectric field. Black paint is applied to the outer surface of a rearsubstrate in the cell to provide a light-absorbing layer outside of thearea defined by the intersection of segment lines and scanning lines.

Cholesteric liquid crystals reflect a portion of the visible spectrumwhen in a reflective state. It is preferable that the reflective statehave neutral color balance. It would be useful to provide cholestericdisplays exhibiting neutral density in the reflective state. It would beuseful for such display to be fabricated using simple, low-costprocesses.

SUMMARY OF THE INVENTION

It is an object of this invention to provide displays that generate alight reflection that is a substantially color neutral when the liquidcrystal in such displays are in the bright state.

It is a further object of the invention to provide a complementarycolored layer that operates in conjunction with a reflective surface,preferably a reflective surface of an electrode, to create acolor-neutral reflective image area in the display.

These objects arc achieved in a display sheet comprising:

-   -   a) a substrate for carrying layers of material;    -   b) an imaging layer comprising polymer-dispersed cholesteric        liquid-crystal material, such imaging layer having a first        relatively higher reflection state within a portion of the        visible light spectrum defining an operating spectrum and a        second relatively less reflective state in said operating        spectrum, wherein said states are capable of being changed by an        electric field between the two states, which states are capable        of being maintained as a stable state in the absence of an        electric field;    -   c) a first transparent conductor disposed on one side of the        imaging layer;    -   d) a complementary light-absorbing layer, on the other side of        the imaging layer, that provides relatively lower transmission        of light within the operating spectrum of the imaging layer and        relatively greater transmission of light outside of the        operating spectrum; and    -   e) a reflective surface, optionally part of a second electrode,        that is capable of reflecting light transmitted through the        complementary light-absorbing layer back through the        complementary light-absorbing layer; and    -   f) a second electrode.

The present invention provides a bright, color-neutral image area whenthe image area is in the planar state. The display can be formed usingsimple, room-temperature processes. Sub-micrometer particles of pigmentin a binder provide an electrochemically stable light-absorbing materialas a thin layer having little effect on drive voltages. Thecomplementary light-absorbing material, comprising a pigment and/or dye,can be coated simultaneous with a polymer-dispersed cholesteric liquidcrystal, preferably an aqueous binder-dispersed cholesteric liquidcrystal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective section view of a first polymer-dispersedcholesteric liquid-crystal display in accordance with the presentinvention;

FIG. 2 is a schematic sectional view of a cholesteric (“chiral-nematic”)liquid-crystal material in a planar and focal-conic state and respondingto incident light consistent with prior-art displays;

FIG. 3 is a sectional view of a domain containing cholesteric liquidmaterial;

FIG. 4 is a plot of the spectral distribution of polymer-dispersedcholesteric liquid crystal with varying domain sizes;

FIG. 5 is a schematic sectional view of optical states of a display inaccordance with the present invention;

FIG. 6 is a the 1931 CIE color matching function of the human eye;

FIG. 7 is a plot of the human eye response to a theoretical cholestericliquid-crystal having a peak reflection of 582 nanometers;

FIG. 8 is the optical response of a display sheet in the planar state inaccordance to the present invention;

FIG. 9 is the optical response of the display sheet of FIG. 8 in thefocal-conic state;

FIG. 10 is a plot of the reflection of light from one embodiment of adisplay sheet, when tested, according to the present invention; and

FIG. 11 is a plot of the red, green and blue components of the displaysheet of FIG. 10 in the planar state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective section view of one embodiment of a firstpolymer-dispersed cholesteric display in accordance with the presentinvention. A sheet designated as display 10 includes a display substrate15, which in one embodiment can be a thin transparent polymericmaterial. One such material is Kodak Estar® film base formed ofpolyester plastic that has a thickness of between 20 and 200micrometers. For example, the display substrate 15 can be a125-micrometer thick sheet of polyester film base. Other polymers, suchas transparent polycarbonate, can also be used.

One or more first conductors 20 are formed on display substrate 15.First conductors 20 can be tin-oxide, indium-tin-oxide (ITO), orpolythiophene, with ITO being the preferred material. Typically thematerial of first conductors 20 is sputtered or coated as a layer overdisplay substrate 15 and has a resistance of less than 1000 ohms persquare. First conductors 20 can be formed in a conductive layer, forexample, by conventional lithographic or laser etching means.Transparent first conductors 20 can also be formed by printing atransparent organic conductor such as PEDT/PSS, PEDOT/PSS polymer, whichmaterials are sold as Baytron® P by Bayer AG Electronic Chemicals.

Cholesteric liquid-crystal layer 30 overlays first conductors 20. Thecholesteric liquid-crystal layer 30 can contain cholestericliquid-crystal material such as those disclosed in U.S. Pat. No.5,695,682 issued Dec. 9, 1997 to Doane et al., the disclosure of whichis incorporated by reference. Such materials are made using highlyanisotropic nematic liquid-crystal mixtures and adding a chiral dopingagent to provide helical twist, in the planes of the liquid crystal, tothe point that interference patterns are created that reflect incidentlight. Application of electrical fields of various intensity andduration can be employed to drive a chiral-nematic (cholesteric)liquid-crystal material into a reflective state, into neartransparent/transmissive state, or into an intermediate state. Thesematerials have the advantage of having first and second optical statesthat are both stable in the absence of an electrical field. Thematerials can maintain a given optical state indefinitely after thefield is removed. Cholesteric liquid-crystal materials can be formedusing a two-component system such as MDA-00-1444 (undoped nematic) andMDA-00-4042 (nematic with high chiral dopant concentrations) availablefrom E.M. Industries of Hawthorne, N.Y.

In a preferred embodiment, cholesteric liquid-crystal layer 30 is acholesteric liquid-crystal material dispersed in gelatin, preferablydeionized photographic-grade gelatin. For example, the liquid-crystalmaterial is mixed at 8% cholesteric liquid crystal in a 5% gelatinaqueous solution. The mixture is dispersed to create an emulsion having8-10 micrometer diameter domains of the liquid crystal in aqueoussuspension. The domains can be formed using the limited coalescencetechnique described in copending U.S. patent application No. 09/478,683filed Jan. 6, 2000 by Stephenson et al. The emulsion can be coated overfirst conductors 20 on a polyester display substrate 15 and dried toprovide an approximately 9-micrometer thick polymer dispersedcholesteric coating. Other organic binders such as polyvinyl alcohol(PVA) or polyethylene oxide (PEO) can be used in place of the gelatin.Such emulsions are machine coatable using coating equipment of the typeemployed in the manufacture of photographic films. A gel sublayer can beapplied over first conductors 20 prior to applying cholesteric layer 30as disclosed in U.S. Pat. No. 6,423,368 by Stephenson et al.

FIG. 2 is a schematic diagram of a chiral-nematic liquid-crystalmaterial, respectively, in a planar and focal-conic state responding toincident light in accordance with prior-art knowledge. In the figure onthe left, after a high voltage field has been applied and quicklyswitched to zero potential, the liquid-crystal molecules align as planarliquid crystals 72, which reflect portions of incident light 60 asplanar reflective light 62. The chiral dopant concentration defines thepeak reflection. The bandwidth around the peak reflection isproportional to the optical birefringence of the nematic liquid crystal.In the figure on the right side of FIG. 2, an application of a lowervoltage field causes molecules of the chiral-nematic liquid-crystalmaterial to break into tilted cells known as focal conic liquid crystals74. The orientation of the focal-conic material is nearly transparent orscattering/transmissive rather than reflective. Changes in thelow-voltage time duration permits molecules to assume orientationsbetween reflective planar state 72 and the light scattering focal conicstate 74.

In the fully evolved focal-conic state 74, the cholesteric liquidcrystal is light scattering and incident light 60 is forward scatteredand can be absorbed by a light absorber 42 to create the appearance of ablack (or blackish-dark) image area. Progressive evolution from a planarto focal-conic state causes a viewer to perceive a bright planarreflective light 62 that transitions to black as the cholestericmaterial changes from reflective planar state 72 to a fully evolvedlight-scattering focal-conic state 74. When the field is removed,cholesteric liquid-crystal layer 30 maintains a given optical stateindefinitely. The states are more fully discussed in U.S. Pat. No.5,437,811 issued Aug. 1, 1995 to Doane et al.

FIG. 3 is a sectional view of a domain containing cholestericliquid-crystal material. Cholesteric liquid-crystal material anchorsagainst an arcuate surface. Incident light 60 can strike the domain atoblique angle 60′ or a relatively perpendicular angle 60. Light strikingcholesteric material at an oblique angle reflects light at a shorterwavelength. The peak reflected wavelength and bandwidth of light is afunction of both the cholesteric liquid-crystal material properties anddomain size and shape.

FIG. 4 is a plot of the spectral distribution of polymer-dispersedcholesteric liquid crystal with varying domain sizes as measured by aspectrophotometer. The cholesteric liquid crystal measured was MerckBL-118, which has a peak reflection of 550 nanometers when measuredbetween two flat glass slides. The cholesteric liquid-crystal materialwas dispersed in a gelatin-containing water bath, and a constantquantity was coated and dried over ITO conductors. A second conductorwas printed over the dried cholesteric coating, and the material wasswitched into the planar (reflective) state to measure reflectance.Spectral distribution was measured for domains having diameters of 2.9,5.0, 8.2 and 10.0 micrometers as emulsions. The domains flatten whendried, which have a surface of varying tilt. As domain size increased,the planar reflected light increased. As evident by FIG. 4, the spectraldistributions had a 5 nanometer shift in peak reflectance to shorterwavelength due to anchoring of the planar liquid crystal on an arcuatesurface reflecting shorter wavelengths of light, which may be taken intoaccount when designing displays. Preferably, the display sheet comprisespolymer-dispersed cholesteric liquid-crystal material with a peakreflected wavelength between 570 and 590 nanometers.

Returning to FIG. 1, complementary light-absorbing layer 35 overlayscholesteric liquid-crystal material 30. In the preferred embodiment,complementary light-absorbing layer 35 is composed of pigments that aremilled below 1 micrometer to form “nano-pigments” in a binder. Suchpigments are very effective in absorbing wavelengths of light in verythin (sub-micrometer) layers. Such pigments can be selected to beelectrically inert to prevent degradation and interference withelectrical display fields applied to display 10. Such pigments aredisclosed in copending U.S. patent application No. 10/222,396 filed Aug.16, 2002, hereby incorporated by reference. The filter layer cancomprise two or more differently hued pigments.

In the preferred embodiment, complementary light-absorbing layer 35absorbs the majority of light normally reflected by liquid crystals inthe planar state and transmits a portion of light not reflected bycholesteric liquid-crystal layer 30. Complementary light-absorbing layer35 should be as thin as possible to minimize drive voltage whileproviding an acceptable degree of light absorption. Pigments areextremely efficient light absorbers and ideally suited for this purpose.In the preferred embodiment, cholesteric liquid-crystal layer 30 isbetween 4 and 10 micrometers thick. The state changing field forcholesteric liquid-crystal materials is typically 10 volts permicrometer coating thickness. Because complementary light-absorbinglayer 35 is disposed between the two field-carrying conductors, thelayer should be significantly thinner than the cholestericliquid-crystal layer 30. In practice, complementary light-absorbinglayer 35 should be less than about 1 micrometers, preferably 0.5micrometers or less in thickness. The amount of binder in complementarylight-absorbing layer 35 should also be low to minimize any increase indrive voltage. It was found that a gelatin binder at a 1:1 ratio withthe pigment can provide a layer with good bond strength to subsequentlyapplied layers and minimize increases in drive voltage.

In the present invention, complementary light-absorbing layer 35 iscoated over cholesteric liquid-crystal layer 30 to provide alight-absorbing layer 35 that provides a high-contrast dark image areain the focal conic state relative to planar reflective light.Complementary light-absorbing layer 35 further provides in the planarstate (via selective transmission of light that can be reflected by anunderlying reflective surface) a pre-designed amount of light atwavelengths not operated on by the cholesteric liquid crystal. Thecomplementary light-absorbing layer 35 can be coated simultaneously withthe deposition of cholesteric liquid-crystal layer 30 or in a separatestep. In a preferred embodiment, multi-layer coating equipment such asused in making photographic imaging elements provides cholestericliquid-crystal layer 30 and complementary light-absorbing layer 35 astwo co-deposited layers. Complementary light-absorbing layer 35 issignificantly thinner than cholesteric layer 30 and, therefore, asmentioned above, has minimal effect on the electrical field strengthrequired to change the state of the cholesteric liquid-crystal materialin the manufactured display.

Continuing to refer to the embodiment of FIG. 1, second conductors 40overlay complementary light-absorbing layer 35. Second conductors 40have sufficient conductivity to induce an electric field acrosscholesteric liquid-crystal layer 30, which field is strong enough tochange the optical state of the polymeric-dispersed liquid-crystalmaterial. Second conductors 40 in this embodiment are formed ofreflective metal, for example, by vacuum deposition of conductive andreflective material such as aluminum, silver, chrome or nickel. In thecase of vacuum coated second conductors 40, aluminum or silver providevery high reflectance and conductivity. The layer of conductive materialcan be patterned using well known techniques of photolithography, laseretching or by application through a mask. Alternatively, a thinreflective layer of a material can be applied before application of aless reflective conductive material to maximize reflection.

In another embodiment, second conductors 40 are formed by screenprinting a reflective and conductive formulation such as UVAG® 0010material from Allied Photochemical of Kimball, Mich. Such screenprintable conductive materials comprise finely divided silver in anultraviolet curable resin. After printing, when the material is exposedto ultraviolet radiation greater than 0.40 Joules/cm², the resin willpolymerize in about two seconds to form a durable surface. Screenprinting is preferred to minimize the cost of manufacturing the display.Providing a sufficient amount of polymer to pigment in complementarylight-absorbing layer 35 creates a printable surface on secondconductors 40.

Alternatively, second conductors 40 can be formed by screen printingthermally cured silver-bearing resins. An example of such a material isAcheson Electrodag® 461SS, a heat-cured silver ink. The first and secondconductors can be patterned to produce an addressable matrix.

FIG. 5 is a schematic section view of optical states of a display inaccordance with the present invention. The left diagram demonstrates theoptical path when the cholesteric liquid-crystal material is in theplanar state. Incident light 60 strikes planar liquid crystal 72 whichreflects a portion of the incident light 60 as planar reflective light62. The remaining light passes through complementary light-absorbinglayer 32. A portion of the light passing though complementarylight-absorbing layer 32 is absorbed, and the remaining light strikesreflective second conductor 40. Light is reflected from second conductor40, passes through complementary light-absorbing layer 32 a second time,and then passes through planar liquid-crystal material 72 to becomecomplementary light 64. On the right side of FIG. 5, the liquid-crystalmaterial is in the focal-conic state and complementary light-absorbinglayer 32 absorbs wavelengths of light reflected in planar state 72.Light outside cholesteric liquid-crystal substantially reflectivewavelengths continues to provide complementary light 64 when theliquid-crystal material is in focal conic 74.

FIG. 6 is a plot of the response of the three color “channels” (red,green and blue) of the human eye, the “channels” referring to thedifferent types of photoreceptors (cones) contained in the retina of thehuman eye. The plot is taken from the 1931 CIE Standard Colorimetricobserver. Light reflected from display accordingly to the presentinvention optimally has about equal excitation (area under the curve) ofall three color channels and, therefore, appears neutral (white/grey).Cholesteric liquid crystals have about a 100 nanometer bandwidth. It ispreferable to select a peak wavelength in that region to maximizebrightness. The peak wavelength that has the greatest excitation forsuch a bandwidth is at approximately 580 nanometers. At that wavelength,excitation of the green (G) and red (R) receptors overlap and arecommonly excited by light at wavelengths between 530 and 630 nanometers.

FIG. 7 is a plot of the human-eye response to a cholesteric liquidcrystal having a peak reflection of 582 nanometers and a 100-nanometerbandwidth. The excitation of the green and red channels, as expressed bythe area under the green (G) and red (R) curves is about equal. If anequal area of blue light is presented to a viewer, then the total lightwill have a neutral (white/grey) hue.

FIG. 8 is the optical response of one embodiment of a display sheet inthe planar state in accordance with one embodiment of the presentinvention. The peak reflection of the planar state of the liquid crystalis set so that light exciting both green (G) and red (R) portions of thevisual spectrum are excited. Complementary light-absorbing layer 35 canbe a coating comprising one or more pigments/dyes that is designed toreflect a portion of blue (B) light to create light that yields asubstantially neutral or whitish color (W). When the liquid-crystalmaterial is in the planar state, the total reflected light, accordingly,comprises both (1) the liquid-crystal-derived reflected light and (2)the light transmitted by the light-absorbing layer and reflected by thereflective surface. The latte reflected light can also be referred to asthe color-filtered second reflected light, since the light-absorbinglayer can functions as a color filter.

FIG. 9 is the optical response of the display sheet of FIG. 8 when theliquid crystal is in the focal-conic state. Complementarylight-absorbing layer 35 absorbs substantial amounts of light ofwavelengths of light, in this case red (R) and green (G), reflected inthe planar state. A portion of the blue (B) light is reflected to createa blue or bluish image layer. The display thereby can switch between aneutral and dark-blue state by electrically switching a cholestericliquid-crystal material between the planar and focal-conic state.

Returning to FIG. 8, a component of short-wavelength light, or blue (B)light, is added by the combination of complementary light-absorbinglayer 35 and reflective second conductor 40. Referring to FIG. 6, it canbe seen that the addition of the short-wavelength blue (B) light furtherexcites red (R′) receptors, increasing apparent red (R) color channelexcitation. The peak wavelength of the cholesteric liquid-crystalmaterial must, therefore, be adjusted to a shorter wavelength toincrease the green light and decrease the red light, so that aneffectively neutral display is created in the planar state with theaddition of the blue light. The blue light component shouldapproximately or substantially match the perceived intensity of thegreen and red components of light and provide a small amount ofexcitation to balance the red against the superior green intensity.Pigments suitable for the application are imperfect, and the finalformulation for the various components is established by trial and theuse of mathematical models.

In a preferred embodiment, reflected light from the display sheet whenthe imaging layer is in the first relatively higher reflection state hasCIE tristimulus values X, Y and Z that are within 20 percent of eachother. It is also preferred that when the liquid-crystal material is inthe planar state, the total reflected light from the display sheet hasCIE tristimulus values X, Y and Z that are 20 percent closer to eachother than the CIE tristimulus values of the liquid-crystal-derivedlight alone without the color-filtered second reflected light. The X, Y,and Z values can be approximated based on measurements made on atristimulus calorimeter R, G, B measurements, as will be understood bythe skilled artisan.

EXAMPLE

An experiment was conducted to create a sheet in accordance to thepresent invention. The sheet was constructed using 300 ohm per squareITO over a 150 micrometer thick polyester sheet. A polymer-dispersedliquid-crystal cholesteric material in accordance with the previouslydescribed formulation was blended to have a peak wavelength of 575nanometers. The cholesteric liquid-crystal material was dispersed in 8.3micron domains and coated over the ITO conductor.

A complementary dye solution was made having 1.74% dissolved gelatin,0.74% Sunfast Blue 15:4 milled to a 110 nanometer mean diameter and0.1.55% Pigment Violet 29 milled to a 210 nanometer mean diameter. Thesolution was coated over the polymer-dispersed cholesteric liquidcrystal at 10.76 grams per square meter and dried to form acomplementary light-absorbing layer. The dried layer was less than 0.5microns thick. Second conductors were printed over the complementarylight-absorbing layer using such as UVAG® 0010 resinous material fromAllied Photochemical of Kimball, Mich. After printing, the resinousmaterial was exposed to ultraviolet radiation greater than 0.40Joules/cm² to form a durable surface.

Prior experiments had determined that polymer-dispersed cholestericliquid-crystal material in the experimental formulation reflectedapproximately 25% of light at 575 nanometers when the material was inthe planar state. The effective reflectivity of the printed silver wasfound to be uniformly 65 percent across the visible spectrum. The dyeconcentration was selected so that the passage of light through thecomplementary light-absorbing layer, reflected by the printed silversecond conductor and back through the complementary light-absorbinglayer would have approximately 25% reflected blue light, as measured atthe peak reflected wavelength of 450 nanometers.

FIG. 10 is a plot of the reflection of light from a test sheet for adisplay according to the present invention. In the planar state (P), thepeak planar reflected light at 575 nanometers matched the reflected bluelight from the complementary light-absorbing layer. When the test sheetwas written into the focal-conic (FC) state the cholestericliquid-crystal material is near transparent/transmissive, and the FCspectrum is a functional plot of the complementary light-absorbing layerworking in combination with the printed silver reflector. Thecomplementary light-absorbing layer is composed of cyan and magentapigments which together have a high absorption of light in the spectrumreflected when the cholesteric liquid-crystal material is in the planarstate. The magenta pigment, Pigment Violet 29 has a peak absorption atabout 540 nanometers, and the cyan pigment, Sunfast Blue 15:4 has a peakabsorption at 620 nanometers. It can be seen in FIG. 10 that thecombination of the two pigments provides 95% light absorption from 525-to 635-nanometer wavelengths. The pigments have very high absorption inthe areas that overlap, and peak at about 575 nanometers. The peak ofreflected light, 25%, occurs in the blue region at about 450 nanometersto balance light reflected in the green and red portions of the visiblespectrum. Portions of the visible spectrum not operated on by thecholesteric liquid-crystal material, in this case the blue portion, areconstant reflectance regardless of the state of the cholestericliquid-crystal material.

FIG. 11 is a plot of the red, green and blue components of the sheet ofFIG. 10 in the planar state, as perceived by the human eye. The areasunder each color curve were approximately equal for all three colorchannels, and the sheet, therefore, had a neutral, light greyappearance. When the test sheet was written into the focal-conic state,the material appeared to be a light blue. The visible difference betweenthe two states was pleasing.

The invention can be practiced substituting materials and processesdifferent from those used to generate the test sheet. Evaporated metalscan be used in place of the printed silver. Displays are improved byusing higher reflecting materials such as aluminum. The complementarypigments used in the test sheet can be replaced by one or morecombinations of pigments or dyes to provide higher absorption of lightin the planar reflective wavelengths while maintaining a constant bluereflection.

Displays can be made according to the present invention to comprisecontrasting colors other than blue-neutral or blue-whitish, as will beunderstood by the skilled artisan. For example, magenta-white,red-white, and yellow-white combinations can be obtained, wherein thehigh reflectance state is “white” or whitish, meaning bright andsubstantially neutral and by the non-white colors is meant a darkertinted color. The peak wavelength of the cholesteric liquid-crystalmaterial can be changed to provide selective reflection and transmissionin other areas of the visible spectrum. For example, the peak wavelengthcan be adjusted to reflect only in the blue- green or intermediatewavelengths between the two colors. The cholesteric liquid-crystalmaterial can be set to reflect primarily red light. In each case,complementary light-absorbing materials can be selected to provide highabsorption in the wavelengths reflected in the planar state, andinvariant portions for other wavelengths. In certain cases, the displaycan provide two contrasting colors/states neither of which are neutral.Usually in practice, however, it is preferable to have a neutral statethat is as bright as possible. A cholesteric liquid-crystal materialnear 580 nanometers with a complementary blue pigment and a reflectivesecond conductor provides a comparatively bright neutral contrast.

Processes can be used to create displays with complementarylight-absorbing layers and reflective conductors using rigid substratesor multiple substrates. Displays can be built using this invention witha substrate moved to behind the reflective second conductor.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 display-   15 display substrate-   20 first conductors-   30 cholesteric layer-   35 complementary light-absorbing layer-   40 second conductors-   42 light absorber-   60 incident light-   62 planar reflected light-   64 complementary light-   72 planar liquid crystal-   74 focal conic liquid crystal-   P Planar state-   FC Focal-Conic state-   R Red or Tri-Stimulus X reflectance-   R′Red excitation by short wavelength-   G Green or Tri-Stimulus Y reflectance-   B Blue or Tri-Stimulus Z reflectanc

1. A display sheet comprising: a) a substrate for carrying layers ofmaterial; b) an imaging layer comprising polymer-dispersed cholestericliquid-crystal material, such imaging layer having a first relativelyhigher reflection state within a portion of the visible light spectrumdefining an operating spectrum and a second relatively less reflectivestate in said operating spectrum, wherein said states are capable ofbeing changed by an electric field between the two states, which statesare capable of being maintained in the absence of an electric field; c)a first transparent conductor disposed on one side of the imaging layer;d) a colored complementary light-absorbing layer on the other side ofthe imaging layer having relatively lower transmission of light withinthe operating spectrum of the imaging layer and having relativelygreater transmission of light outside of the operating spectrum; and e)a reflective surface, optionally part of a second electrode, thatreflects light transmitted through the colored complementarylight-absorbing layer back to the complementary light-absorbing layer,wherein the colored complementary light-absorbing layer is capable ofoperating in conjunction with the reflective surface to provide asubstantially neutral reflective image when the imaging layer is in thefirst relatively higher reflection state; and f) a second electrode. 2.The display sheet of claim 1 wherein reflected light from the displaysheet when the imaging layer is in the first relatively higherreflection state has CIE tristimulus values X, Y and Z that are within20 percent of each other.
 3. The display of claim 1 wherein the displaysheet comprises polymer-dispersed cholesteric liquid-crystal materialwith a peak reflected wavelength between 570 and 590 nanometers.
 4. Thedisplay of claim 1 wherein the colored complementary light-absorbinglayer comprises sub-micrometer pigment particles in a polymeric binder.5. The display of claim 1 wherein the reflective surface is on thesecond conductor that is formed of printed silver ink.
 6. The display ofclaim 1 wherein the reflective surface is on the second conductor thatis formed of vacuum deposited metal.